Capacitors have been used by electric power utilities for many years to correct and adjust the power factor in alternating current power distribution systems. When highly inductive loads, typically large motors used in industrial plants, are connected to power lines, a shift in the phase angle between the supplied voltage and current results. Because the total instantaneous power in a three phase electric power system is related to the cosine of the phase angle between the line voltage and line current, the difference in the phase angle between the line voltage and line current may be expressed as a value called a power factor in which a power factor of 1.0 VARS (volt-ampere reactive) represents a 0 degree phase shift and a power factor of 0.0 VARS represents a 90 degree phase shift. A + or - sign is often used to indicate whether the current leads or lags the voltage respectively.
This phase shift creates two problems for the power company. First, a large amount of reactive power is used in distributing electrical energy on a power line having a small power factor. The power company must supply this power which represents a nonproductive loss of power in the distribution of the electrical energy. As a result, power companies often require consumers of electric power to provide for a minimum power factor at their point in the power distribution network as a part of their power consumption responsibilities. Second, a small power factor and the above-mentioned associated line losses result in a drop in the line voltage at the customer site. This is problematic to the customer equipment that relies on a reliable and steady line voltage.
To solve these problems, a well-known practice of introducing capacitor banks on the power lines near the load has been employed. One solution is to permanently connect a fixed amount of capacitance to the line. A better solution is to provide several banks of capacitors that are switched in and out, depending on the power factor, or line voltage. The decision to switch the capacitors in or out can vary between simple algorithms, which take into account the time of day, temperature or voltage (often used where air-conditioning loads are the main problem), to extremely complex algorithms, which combine the actual phase shift, temperature, voltage, time of day, available capacitors, and the actual load, in addition to other power distribution variables. In either case, the net effect of introducing this capacitance is to reduce or eliminate the reactive power and/or voltage losses caused by the inductive motor loads and to return the power factor closer to unity.
Power factor correction at the electric power generating facilities themselves is known. As illustrated in U.S. Pat. No. 4,645,997, large solid state switches are often used with complicated algorithms to introduce the corrective capacitance onto the line at appropriate times to avoid voltage transients. In particular, unless the corrective capacitors are switched in at the voltage zero-crossings or out at the current zero-crossings, a voltage surge or transient is produce that disrupts the normal line voltage. Over the past several years, factories have increasingly installed very expensive, complex motor control systems that increase efficiencies and allow better speed control. Further, a significant increase in the industrial use of computer systems and other sensitive equipment has taken place in the last few years. Both of these systems rely heavily on the control of the supplied electric power during sub-cycle times and are easily disrupted by voltage surges or transients caused by switching capacitors in or out in the middle of the cycle, i.e. not at the appropriate zero-crossings.
Expensive solid state switches, used at power generation facilities to connect the corrective capacitors, have fast switching times as compared to the 60 Hz. power cycle period of 16.66 milliseconds. These fast switches simplify the calculations involved in properly introducing corrective capacitors onto the power line at appropriate times. That is, the switch delays are often not a parameter to include in the calculation of zero-crossing switch times. In this regard, the switch should be closed within about 2 microseconds of the zero-crossing when connecting the capacitor to the line to minimize line voltage disturbances.
However, the electric power distribution system also includes thousands of corrective capacitor banks at various pole-mounted locations along the power distribution network. These power distribution points typically handle 15-20 kilovolts (KV) power lines. Traditionally, the power companies have remotely administered the connection of these capacitors through the use of programmable circuit boards atop the poles. Unlike the capacitor banks with solid state switches in the power generation facilities, however, less expensive, solenoid driven switches are used to connect these remotely located corrective capacitors to the power lines. Unlike their solid state counterparts, these solenoid driven switches have large switch times compared to the power cycle period, often on the order of 10 milliseconds or more. As such, the switching of these solenoid driven switches has heretofore taken place without any consideration of the line voltage at the instant of closure. In addition to the large switch time that must be taken into account, these solenoid driven switches are also prone to mechanical degradation over time. This degradation alters the switching time of the device and increases the timing of when to switch the capacitors in and out. Further, the closing and opening times of the switches may be different from one another and must therefore be accounted for appropriately in the calculation of the switch's opening and closing.
Thus, the need exists for a synchronous capacitor switch controller that provides disturbance feedback to accurately and reliably determine and control an appropriate switching time for a solenoid driven or mechanical switch having a long switching time relative to the power line power cycle period. Further, the need exists for a switch controller that accounts for a switching time that changes over time. Also needed is a switch controller that uses only a voltage monitoring of line disturbances to determine is switching times thereby obviating the need for expensive current transformers to detect capacitor currents.